Already known are control devices which comprise a valve case having a fluid channel, a casing provided on an upper portion of the valve case, a valve stem disposed inside the casing and reciprocatingly movable upward and downward to thereby cause a valve element to open or close the fluid channel, a piston upwardly or downwardly movably provided inside the casing and defining a compressed air admitting space along with a top wall of the casing, and booster means for transmitting the force acting on the piston to the valve stem upon amplification (see, for example, the publication of JP-A No. 07-139648).
Control devices having such booster means are termed high-temperature valves and used at a temperature of at least 150° C. in some cases, but still remain to be improved in heat resistance and have problems with respect to sealability and durability especially for use at high temperatures of about 300° C.
An object of the present invention is to provide a control device having booster means and usable at high temperatures.